The high pressure behaviors of hydrate Cu-BTC metal-organic framework (MOF) in terms of phase stability, compressibility and reversibility were investigated in situ by synchrotron X-ray powder diffraction as well as vibrational spectroscopy. Two phase transitions, caused by the interaction of water and the sample framework, were revealed by the vibrational spectroscopies. Compressibility of the hydrate Cu-BTC also displays soft and hard regimes, which is the same scenario as non-hydrate Cu-BTC with a pressure transmitting medium. It is further confirmed that the residual water molecules in hydrate Cu-BTC can serve as a pressure transmitting medium with small molecule size under high pressure. Our results not only prove the high stability of Cu-BTC but also provide spectroscopic evidence for the interactions taking place between the guest molecules and the sample framework. Such findings could provide further guidelines for improving Cu-BTC's absorption and storage abilities.
Knowledge of the high-pressure behavior of carbon dioxide (CO 2 ), an important planetary material found in Venus, Earth, and Mars, is vital to the study of the evolution and dynamics of the planetary interiors as well as to the fundamental understanding of the C-O bonding and interaction between the molecules. Recent studies have revealed a number of crystalline polymorphs (CO 2 -I to -VII) and an amorphous phase under high pressure-temperature conditions. Nevertheless, the reported phase stability field and transition pressures at room temperature are poorly defined, especially for the amorphous phase. Here we shed light on the successive pressure-induced local structural changes and the molecular-to-nonmolecular transition of CO 2 at room temperature by performing an in situ study of the local electronic structure using X-ray Raman scattering, aided by first-principle exciton calculations. We show that the transition from CO 2 -I to CO 2 -III was initiated at around 7.4 GPa, and completed at about 17 GPa. The present study also shows that at ∼37 GPa, molecular CO 2 starts to polymerize to an extended structure with fourfold coordinated carbon and minor CO 3 and CO-like species. The observed pressure is more than 10 GPa below previously reported. The disappearance of the minority species at 63(±3) GPa suggests that a previously unknown phase transition within the nonmolecular phase of CO 2 has occurred. mineral physics | diamond anvil cell | inelastic x-ray scattering M olecular compounds such as N 2 and H 2 O have been known to acquire a nonmolecular structure under compression and ultimately transform into highly disordered and/or amorphous phases (1-4). Other solids, such as group IV oxides SiO 2 (5) and GeO 2 (6), are prone to amorphize under pressure despite the covalent framework structure. However, it was not until recently that CO 2 , both a molecular compound and group IV oxide, was also reported to form several polymorphs and a pressureinduced amorphous phase (7-23). Previous experimental evidence on the formation of nonmolecular phases of N 2 and CO 2 was mainly based on the loss of optical vibrons (1) and/or the emergence of a broad IR or Raman band in the stretching mode region. Unfortunately, these new features are often very weak and accurate measurement was hindered by significant noises (17,22,23), making it difficult to extract useful information on the local coordination in the amorphous phase. This difficulty is illustrated by studies of the high-pressure high-temperature amorphous a-CO 2 phase in which the polymeric local structure was claimed to be a mixture of tetrahedral and octahedral coordinated carbon based on Raman spectroscopy (17), but to a mixture of tetrahedral and threefold coordinated polymeric CO 2 from a combined theoretical and infrared spectroscopy study (20). Moreover, knowledge of the phase diagram and kinetics for the various phase transitions in solid CO 2 are obscured by significant disparities in the estimated phase transition pressures from different studies and tec...
The strength and texture of sodium chloride in the B1 (rocksalt) and B2 (cesium chloride) phases were investigated in a diamond anvil cell using synchrotron X-ray diffraction in a radial geometry to 56 GPa. The measured differential stresses within the Reuss limit are in the range of 0.2 GPa for the B1 phase at pressure of 24 GPa and 1.6 GPa for the B2 phase at pressure of 56 GPa. A strength weakening is observed near the B1-B2 phase transition at about 30 GPa. The low strength of NaCl in the B1 phase confirms that it is an effective pressure-transmitting medium for high-pressure experiments to ∼30 GPa. The B2 phase can be also used as a pressure-transmitting medium although it exhibits a steeper increase in strength with pressure than the B1 phase. Deformation induces weak lattice preferred orientation in NaCl, showing a (100) texture in the B1 phase and a (110) texture in the B2 phase. The observed textures were evaluated by viscoplastic self-consistent model and our results suggest {110}⟨11¯0⟩ as the slip system for the B1 phase and {112}⟨11¯0⟩ for the B2 phase.
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